Auto AC cross charged TXV, why different SH vs P curves?

[AliXV]

I have two block style TXVs from Japanese-style Kei cars.
They look visually identical but they have two different control charges.
Both cars come with the same evaporator, same engine displacement, both 1.2-1.3L petrol engines.
One of them has a tiny 70/100cc fixed displacement Seiko Seiki vane compressor, the other has a bigger 125cc max displacement variable swashplate Delphi CVC comp.
The TXV for the former (vane) seems to have a higher MOP setting and generally speaking seems to flow less refrigerant in the evaporator since it fully closes after cooling the power head with freeze spray and takes a short while to reopen again, while the other TXV (variable displacement comp) only throttles a bit even with a frozen power head, and takes a longer while to open wider, so overall it seems to flow more refrigerant and has a higher MOP.

After further research i found that, basically, auto TXVs all belong to the cross charged type and all go 0 superheat at low heat loads since they're designed to be still open around 32F evap temp, however according to the compressor type they provide around 5K or 10K superheat at higher heat loads and accordingly different refrigerant flows.

Vane compressors-10K "hot"
Fixed displacement swashplate-10K hot on small displacement engines, 5K hot on V8s and the like
Variable displacement piston-Nearly always 5K hot except few exceptions (some Toyota Corollas, early Scion Xbs and Scion Xas)
Scroll-Indifferently 5K or 10K hot

I understand more superheat means less efficient evaporator's cooling capacity usage, so i think using a 10K instead of a 5K might mean the AC would take longer to cool.

Any other possible reasons? Smaller compressors aren't meant to pump too much refrigerant perhaps?
Too much engine load on small engines and fixed displacement comps? Too little refrigerant flowing through a cold evap might possibly mean unstable compressor displacement control with variable compressors?

Thanks

[seanf]

Are the heat loads the same? Cabin size, max number of people, number/size of windows.
Are the cars the same maufacturer, model and age, same spec, specd for the same country?
Are they on the same refrigerant?

[AliXV]

Same refrigerant (R134A), same heat loads.

It occurs to me that on one occasion the TXV originally had a 10K "hot" setting and then got subsequently replaced with a new part number which featured a 5K "hot" power head charge. 90cc maximum displacement Denso 5SE09 clutchless comp in that case, same car.

The Corolla equipped with the 10K "hot" TXV and a clutchless 140cc maximum variable comp was marketed in Mexico only as far as i know, otherwise it shipped with a 170cc fixed displacement swashplate and same TXV in the USA.

The Japanese use 10K "hot" TXVs in 0.6L Kei cars with vane compressors. Perhaps a way to limit the cycling clutch "bump"?

I know for sure they all go to 0K SH @ 32F, otherwise variable comps do hunt and oil begins pooling inside the evaporator once the comp "destrokes" due to low heat load.

[AliXV]

Ok, i think i've figured it out somewhat.

The one with the higher superheat and correspondingly lower MOP (spring tension appear to be the same, it's just a matter of how much control charge they load in the powerhead) appears to be better suited for cycling systems since it pulls down evap pressure faster. The price to pay is slightly lower usage of the evap surface at medium heat load due to the lower mass flow. Keep in mind there's a clutch hysteresis.

The one with the lower superheat appears to be better suited for variable displacement applications, where it may happen that the system steadies out at medium load. In that case, more of the evaporator's surface will be exchanging heat (superheated ref doesn't take as much heat away as liquid one), but it will take a little bit more to cool.

In any case, with the fixed displacement com, the 0 superheat at low load and the resulting flooded evap condition helps prevent uneven frost forming on the evaporator coil from the inlet to the outlet as the compressor capacity increases due to the engine being revved up.

Thanks to the cross charge, the evap only freezes up as a whole and that is prevented by the evap temp probe/switch that cycles the clutch off once the air blowing past the evap fins gets too cold.

With a parallel control charge, the TXV would starve the evap and uneven frost formation would ensue due to the lower than normal evap coil pressure and the tiny amount of liquid making it into the evap. Evap temp probe/switch could possibly be unable to function properly.

Evap SH may temporarily go to 0 but there's still some suction superheat keeping the comp safe and sound due to the comp and the suction line being placed inside a hot engine bay.

Wet compression ain't a problem anyway when a variable compressor runs fully destroked.

Does anyone agree?

[seanf]

If youve got a larger capacity compressor youll need a TXV that can deliver higher mass flow rate.

Not sure I would want liquid to get back to any type of automotive AC compressor.

Maybe theres reasons they run with a larger capacity compressor, maybe a higher efficency at partial load, or maybe for a higher initial pull down rate, maybe one car has been designed with the idea it would be running at lower rpm for a greater amount of the time, maybe its to put the designed load condition in the middle of the compressors capacity range so that it can still control if the load goes lower or higher than expected.

[AliXV]

Toyota Prius went from a small clutch equipped scroll to a way larger hermetic electric comp between 03 and 04 MY's.
Accordingly, the cross charged TXV got replaced to one featuring greater mass flow rate.

I can see the larger compressor capacity and the fact it won't cycle anymore (so way more time steady @ part load where the more the evaporator surface being used, the better it is) as the factors here.

TGK seems to have filed a patent for a TXV that basically gives 10K "hot" mass flow rate at high heat load and 5K "hot" like mass flow rate at low heat load thanks to a bimetal spring. See patent JP2010112616A for more info.
I seem to understand they basically want to achieve fast pulldown and stable variable compressor operation/better cooling capacity at the same time.

About liquid getting into the comp, it seems it's a deliberate choice to flood the evap at low heat loads.

Number of reasons, mainly oil return, hunting at low heat loads with variable compressors and frost prevention.
Generally speaking designers want the automotive TXV to act more like an AXV once the evaporator is about to freeze up by keeping it slightly open and ensuring a minimum flow at all times.

There always seems to be some suction superheat, for one i'm sure i have such a TXV and a variable displacement compressor on my own car but never ever seen frost anywhere, be it on the entire suction line or TXV. Even on a fully charged system.

See patents US4632305, US20050066674A1 and US4979372 for more info.